Julie, the top layers are called underlay. The subfloor would be below the oak flooring you wish to expose. Begin in a doorway, where you can have room to pry between the sheets of underlay. Use a flat shovel or similar as a shield against damaging the oak floor, and pry from this with a claw hammer until you've opened it up enough to use a long prybar to get further in onto the sheet. Work it methodically, and often you can get entire sheets to come up, including most of the staples.